Percussion drilling is a well-established technique that breaks rock by hammering impacts transferred from the rock drill bit, mounted at one end of a drill string, to the rock at the bottom of the borehole. The energy needed to break the rock is generated by a hydraulically driven piston that contacts a shank adaptor positioned at the opposite end of the drill string to the drill tool. The piston strike on the adaptor creates a stress (or shock) wave that propagates through the drill string and ultimately to the borehole rock bottom. To achieve maximum drilling efficiency, various physical parameters associated with the piston, the shank adaptor and drill rods of the drill string must be optimised.
In particular, the shock wave created within the drill string typically comprises a rectangular shape profile. The length of the shock wave is twice the axial length of the piston whilst the amplitude is dependent on the velocity of the piston at the moment of impact and a relationship between a cross sectional area of the piston impact end and that of the drill string. The optimised energy is typically achieved by variation of these parameters including piston geometry and impact rate and frequency.
However, the energy within the shock wave typically decreases as it travels axially along the drill string and through each threaded coupling that connects the drill rods. This loss results from differences in the cross sectional area between the male and female threaded couplings involving reflections and impedance transmissions that generally change the shape of the shock wave as it propagates. Depending upon the physical characteristics of the drill string and indeed the piston and shank adaptor, the transmitted wave can be smoothened or increased in amplitude due to super positioning and reflections. Example percussion drilling systems are described in GB 659,331; SE 432280; WO 2008/041906 and U.S. Pat. No. 8,061,434. U.S. Pat. No. 8,061,434 in particular describes a method of controlling operation of the percussion piston to influence the shape of the stress wave in an attempt to increase drilling efficiency.
However, existing percussion drilling systems are not optimised to reduce as far as possible energy loses within the shock wave as it propagates along the entire length of the drill string whilst delivering an energy shock wave at the drill string tool having a shape characteristic is optimised for rock breakage. There is therefore a need for a percussion drilling system that addresses these problems.